To get the kite up, you have to run against the wind. The same is true for airplanes, why is that?
Here we need to distinguish between two speeds of the aircraft - ground speed and air speed. Ground speed is the speed of the aircraft relative to the ground, and airspeed is the speed of the aircraft relative to the surrounding airflow. The take-off of an aircraft relies on the pressure difference generated by the difference in air velocity between the upper and lower sides of the wing, which in turn provides lift. Therefore, it is the airspeed, not the ground speed, that determines whether the aircraft can take off smoothly.
Suppose an airplane needs an airspeed of 300 km/h to take off. If there is a wind blowing at 50 km/h relative to the ground at this time, the aircraft only needs to accelerate to a ground speed of 250 km/h. But if you are flying downwind, the wind speed will cancel the airspeed, and the plane needs to accelerate to a ground speed of 350 km/h.
However, in the cruise phase after takeoff, it is more efficient to fly downwind. Some flights can cut sailing times considerably if they hitch a ride in the atmospheric jet stream. In the final landing stage, it is necessary to increase the resistance to slow down, and it is safer to be headwind at this time.
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